Oscillation circuits are known to be used in phase locked loop circuits (PLLs) and enable the PLL to generate precise frequency outputs based on a given reference signal frequency. As is also known, PLLs are used in a wide variety of applications that require accurate synchronization between signals. For example, processing circuits (i.e., digital signal processors, microprocessors, microcomputers, microcontrollers, etc.) use PLLs to ensure synchronization of circuit elements. Wireless communication devices (such as mobile radios, cellular telephones, and transmission sites) also use PLLs to ensure proper tuning of transmitters and receivers such that a wireless communication can occur. Data recovery systems, such as Asynchronous Transfer Mode (ATM), Asymmetrical Digital Subscriber Line (ADSL), and Integrated Services Digital Network (ISDN), use PLLs to recover clock signals from incoming data streams.
PLLs generally produce an output frequency based on some multiple of the reference signal frequency via a phase-detect stage, a voltage controlled oscillator, and a feedback stage. The phase-detect stage compares the reference signal frequency to an internally generated feedback oscillation signal and generates a frequency adjusting signal based on the phase relationship of these two signals. This frequency adjusting signal regulates the output frequency signal produced by the voltage controlled oscillator (VCO), wherein the output frequency signal is divided by the feedback stage to produce the feedback oscillation signal. As the reference signal frequency changes, the phase-detect stage detects the change and adjusts the frequency adjusting signal such that the output frequency signal remains "locked" with the reference signal frequency.
One prior-an VCO is shown in FIG. 1 to include a varactor, a first capacitor, an inverter, a crystal oscillator, a second capacitor, a buffer, and resistors. In operation, the output frequency, which is provided at the output of the buffer, is adjusted based on the frequency adjusting signal. The frequency adjusting signal varies the capacitance of the varactor, thereby changing the capacitive load at the oscillator, causing the output frequency to be adjusted. While this type of VCO works well for typical supply voltages (5 volts), it does not support low voltage applications (supply voltages less than 3 volts) due to limitations of the varactor.
A second type of VCO is shown in FIG. 2. In this implementation, the frequency is digitally adjusted by controlling a series of switching elements, each with an associated capacitor, to change the capacitance at the oscillator. As switching elements are controlled, the value of their associated capacitors is added, or subtracted, to create a variable capacitance node. As with the varactor implementation, this change in capacitance causes the output frequency to change. One problem with the implementation of FIG. 2 is that the output frequency can only be changed in steps due the step changes of the capacitance values, resulting in a loss of accuracy. A second problem with the implementation of FIG. 2 is that a large amount of space is required for the necessary capacitors; this causes unnecessary system costs.
Therefore, a need exists for an oscillator, which may be incorporated in a PLL, that can operate with a high degree of accuracy at low voltages, without utilizing unnecessary space.